Heart Rate Variability Is Not A Predictor Of Orthostatic Intolerance In Healthy Young Women

Abstract

Various predictors of orthostatic tolerance have been proposed in the literature including gender, height, and training status. More recently, haemodynamic predictors have also been shown to predict tolerance to Lower Body Negative Pressure (LBNP), an effective means of inducing central hypovolemia. However, the literature is not conclusive, suggesting other factors also play a role in the mechanisms responsible for orthostatic intolerance. Heart rate variability (HRV) is thought to be a good index of autonomic control. Studies to date have shown women to have enhanced parasympathetic input to cardiac regulation compared to age-controlled men. It is possible that difference in autonomic balance (HRV) may explain (in part) the increased presence of orthostatic intolerance in women. PURPOSE: Therefore, the purpose of this investigation was to test whether HRV varied between orthostatic and intolerant women. METHODS: Eleven healthy college-age women (Age = 24± 3 (SD) yr) participated in a testing session involving two 12-minute baseline collection periods (pre- and post-LBNP) and four 12-minute stages of progressive LBNP at -15, -30, -45 and -60 mm Hg. Five of the eleven (45%) women became pre-syncopal in the last stage of LBNP (low tolerance = LT). The remaining 6 women were classified with high tolerance (HT). RESULTS: No significant differences existed between HT and LT for any of the measured HVR variables: total power (TP), high frequency (HF), low frequency (LF), LF/HF ratio, SDNN, or RMSSD. Resting LF/HF ratios were comparable to others reported in the literature for healthy individuals (1.32±1.07 and 1.10±.59, for HT and LT respectively). Moreover, there was no direct relationship between any of the time or frequency domain indices of HRV and the rate of decline in SV during the orthostatic stress test. CONCLUSIONS: The results of this investigation do not support HRV as a valid indicator of orthostatic intolerance. Furthermore, individual differences in HRV cannot be used to predict the cardiovascular response to an orthostatic challenge.